Ogata Kōan (緒方 洪庵, August 13, 1810 – July 25, 1863) was a Japanese physician and rangaku scholar in late Edo period Japan, noted for establishing an academy which later developed into Osaka University. Many of his students subsequently played important roles in the Meiji Restoration and the westernization of Japan in the Meiji period. His true name was Ogata Koreaki (緒方 惟章) or Ogata Akira (緒方 章); the name of Kōan was his courtesy name.

In 1838, Ogata returned to Osaka to establish his medical practice, and in the same year established the Tekijuku, an academy of rangaku studies, where he taught medicine, natural history, chemistry and physics for the next 24 years.[2] Ogata used his small but precious collection of Dutch books, including a Dutch-Japanese dictionary and a Dutch encyclopedia, to teach his pupils to read scientific Dutch texts. He also wrote several books, including a treatise entitled How to treat cholera, which he compiled in haste from various European sources during the great cholera epidemic of 1858.

From December 1849, he struggled to gain acceptance of the new smallpoxvaccination, eventually opening 186 vaccination centers from Edo to Kyushu, and obtaining official recognition of the method in 1858. In 1862, Ogata was appointed personal physician to shōgunTokugawa Iemochi, who supported the introduction of western medicine and the establishment of a western medicine research institute in Edo. However, Ogata died a few months later in July 1863 of acute hemoptysis, caused by the tuberculosis he had suffered from for many years.

His house still exists in downtown Osaka. Built in a conventional eighteenth-century style, the students left their mark on the central post of the second-floor classroom, slashing and hacking it with their swords.[3]